This invention relates in general to semiconductor packages and in particular to lead and socket structures with reduced self-inductance.
The leadframes initially used by the electronics industry for semiconductor packaging were made of Kovar (29% nickel, 17% cobalt and 54% iron). For at least the last ten years, Alloy 42 (42% nickel, 58% iron) has largely replaced Kovar for use in leadframes. Alloy 42 was originally formulated as a glass-seal alloy for use as terminal pins on light bulbs and vacuum tubes.
The primary concern in the packaging industry insofar as leadframes are concerned are the mechanical, electrical and thermal properties of the leadframe material. The magnetic properties of these materials have largely been ignored. When a conductor made of ferromagnetic material is used to conduct electric current, where the magnetic material has appreciable magnetic permeability within the frequency of the current, the conductor exhibits a relatively large self-inductance. In other words, on account of the magnetic permeability of the material of the conductor through which current passes, an inductance develops in the conductor which resists the passage of current. High self-inductance causes noise on power and ground lines during simultaneous switching of outputs and causes propagation delays.
With the rapid movement in the electronics industry to increase the density of circuit components in a given area on semiconductor chips, simultaneous switching noises and propagation delays caused by self-inductance become an important issue, particularly in the sub-nanosecond time frame of high-speed very large scale integrated circuit (VLSI) type designs. The self-inductance problem in semiconductor packages has been recognized by the packaging industry. This is indicated, for example, in two articles:
(1) Shrivastava et al., "Inductance Calculation and Optimal Pin Assignment for Pin Grid Array and Chip-Carrier Packages," Abstract, pages 384-391, Phoenix, Arizona;
(2) Hamilton et al., "Molded Carrier Rings for Fine Pitch Surface Mount Packages," Abstract, pages 504-512.
It is therefore desirable to provide leadframes and other structures for use in semiconductor packaging where self-inductance is reduced.